Laser arrays of multiple wavelength sources have many important applications. For example, a color xerographic printer that uses four different wavelength laser beams can have significantly higher throughput than a color xerographic printer that uses only one laser beam. This is so because a four wavelength laser printer can produce overlapping beams, sweep those beams using a single raster output polygon scanner and a single set of optics, subsequently separate the individual beams using wavelength selective filters, and direct each beam onto a separate xerographic imaging station. A latent image for each wavelength is then developed and a full color image is obtained by transferring the developed images onto a single recording medium. In another application, multiple wavelength overlapping beams are imaged without separation at a single imaging station. Once gain, the multiple beams allow higher throughput than a single beam.
A diode laser package with closely spaced emitters would allow a single set of optics to be used, and would eliminate the need for beam combining optics. However, the individual laser diodes in such a package should be closely spaced (preferably within 200 .mu.m) to avoid off-axis distortion effects as the beams propagate through the optical system.
While multiple wavelength laser sources are advantageous, the use of multiple wavelengths creates its own set of problems. For example, the focal length of a laser beam through a given set of optics is wavelength dependent. Thus, if a single set of optics is used in a multiple wavelength system, the different wavelength laser beams will have different focal lengths. In a printer, different focal lengths will result in multiple focal planes for the imaged spots if all laser beams emanate from the same plane. Different focal positions cause various registration problems and are highly undesirable.
One approach to obtaining similarly dimensioned spots is to offset the various laser sources along the optical axes of the overlapped beams such that all beams produce focus in the same plane on their respective photoreceptor(s). In monolithic laser arrays such optical axis offsets are difficult to achieve. However, nonmonolithic laser arrays can be easily offset along the optical axis.
A problem with nonmonolithic laser arrays is the difficulty of mounting the individual lasing elements accurately and closely together. Such mounting becomes even more difficult if, as frequently is the case, electrical, optical, and thermal crosstalk between the individual laser elements must be avoided (or at least reduced to small levels). Adding the requirement of accurate offsets along the optical axis only further complicates the general mounting problem.
Thus, there exists a need for nonmonolithic laser arrays, and techniques that enable such arrays, that have accurately and closely spaced lasing elements, that have low electrical, optical, and thermal crosstalk, and that permit the individual lasing elements to be accurately offset along the optical axis of the system.